The invention relates to a current-source arrangement comprising:
means for generating a plurality of currents which, at least pairwise, are in a ratio which is substantially equal to a given current ratio,
means for detecting deviations in the values of at least some of the currents from those required to realize the given current ratio, and
means for correcting said currents so as to reduce the detected deviations.
In various fields of electronics there is a need for circuits which are capable of supplying currents whose values bear a very accurate ratio to each other. Such circuits are required in, for example, measuring equipment, where various current ratios must be available for measuring-range selection, and digital-to-analog converters which form an analog signal by adding a plurality of currents to each other in accordance with a digital code.
It is known to generate binary weighted currents in a digital-to-analog converter by applying a reference voltage across resistors of binary weighted resistance values or across a so-called resistive ladder network. The accuracy of this type of digital-to-analog converter mainly depends on the accuracy of the resistance ratios. In order to obtain these accurate ratios the resistors are trimmed, for example, by means of a laser. However, in practice, resistor trimming is a time-consuming and expensive operation.
Another known method of generating currents with accurate current ratios employs the dynamic element matching principle. This principle is known from U.S. Pat. No. 3,982,172 and U.S. Pat. No. 4,125,803. These employ a current-source arrangement which supplies a plurality of currents which have substantially equal current values due only to the limited accuracy of the integration process. By means of a permutation circuit, these currents are transferred to outputs of the permutation circuit in accordance with a cyclic permutation. The currents on the outputs each comprise a direct current of a value equal to the average value of the currents from the current-source arrangement and a ripple which is superimposed on the direct current, which ripple is formed by the differences between the currents from the current source arrangement. The ripple can be removed by arranging a filter capacitor on each output of the permutation circuit. However, because of their large values, such filter capacitors cannot be integrated but must be added externally to the integrated circuit. This requires additional connection pins, which entails additional costs. For example, for a 16-bit digital-to-analog converter using such a current-source arrangement 16, additional connection pins are required.
This means that there is a need for methods which enable currents with accurate mutual current ratios to be generated in a simple manner.
A current-source arrangement of a type as set forth in the opening paragraph is known from the article "An Untrimmed D/A Converter with 14 Bit Resolution" IEEE Jnl. of Solid-State Circuits, Vol. SC-16, No. 6, December 1981, pages 616-620. This digital-to-analog converter comprises a ladder network with untrimmed resistors. Deviations in the values of the currents are detected by comparing the output voltages of the digital-to-analog converter which correspond to said currents with a very accurate sawtooth voltage. The information about said deviations is stored in a memory. Subsequently the information is employed for driving a sub-digital-to-analog converter by means of which the deviations in the currents of the digital-to-analog converter are eliminated.
Such a circuit arrangement does not require resistor trimming, but it does require a sawtooth voltage of very high linearity over a wide range, which is difficult to achieve.